1. Field of the Invention
This invention relates to molded case circuit breakers and the moving conductor assemblies in such circuit breakers.
2. Background Information
Molded case circuit breakers include for each pole a fixed contact and a moveable contact. The moveable contact is mounted on a moving conductor assembly which includes a contact arm having the moveable contact affixed at a free end of the arm. The other end of the contact arm is supported by a contact arm carrier for rotation between a closed and an open position of the contacts by a spring powered operating mechanism. The moving conductor assembly includes contact springs which bias the moveable contact against the fixed contact with the contacts closed to provide contact pressure and to accommodate for wear of the contacts. It is common for the contact arm to be made of a stack of copper laminations in which case multiple springs are provided, each biasing one or more of the laminations. Often, the fixed and moveable contacts include main contacts and arcing contacts arranged so that the arcing contacts open after the main contacts and therefore experience most of the wear associated with interrupting the arcs generated by opening the contacts when they are carrying large currents.
It is common in molded case circuit breakers to provide a blow open feature in order to speed response of the circuit breaker to short circuits. The contact arm, or individual laminations, are pivotally connected to the contact arm carrier so that the large magnetic repulsion forces generated by a short circuit current pivot the contact arms on the carrier before the spring powered operating mechanism can rotate the carrier to open the contacts. In many molded case circuit breakers, a separate set of springs, in addition to the springs providing contact pressure, control the level of current required to blow the contact arms open. It is desirable to have a single set of springs to perform both functions, both to save space and to reduce cost. While there are some molded case circuit breakers having a single set of springs to perform both options, the high spring force needed to provide the required contact pressure can place a limitation on the response to a short circuit.
There is a continuing desire to increase the current rating of the various sizes of molded case circuit breakers. Generally, the current rating is a function of the size of the conductors that can be accommodated in a given circuit breaker frame, as the current rating is limited by restrictions on the temperature rise within the circuit breaker. Another concern in the design of molded case circuit breakers is the ease of assembly. All of the various parts of the moving conductor assembly such as the arm laminations, a number of small springs, flexible shunts for connecting the contact arm laminations with a load conductor, and other parts, must be assembled under loading of the springs.
It is possible if the contacts become welded closed, such as by arcing, for the handle on the operating mechanism on some molded case circuit breakers to be moved to the off position, and even locked in the off position, even though the circuit has not been interrupted.
There is room for improvement in all of these aspects of molded case circuit breakers.
Thus, there is a need for an improved circuit breaker and moving conductor assembly which provides the required contact pressure, but which allows the contact arms to blow open quickly and easily in response to short circuit currents.
There is also a need for a moving conductor assembly which provides a capability for increased current rating for a given size circuit breaker frame.
There is also a need for such an improved circuit breaker and moving conductor assembly which can satisfy the above needs and also provide a positive indication when the contacts are welded closed.
There is a particular need for such an improved moving conductor assembly which is simple in construction and easy to assembly to thereby reduce the cost and difficulty of assembly.